Volume rendering is a technique for displaying three-dimensional (3D) data using a classification method called a volume rendering transform (VRT) or transfer function, which maps image intensities to colors and opacities. By varying the parameters of the VRT transfer function, a user can manually cause various structures to become selectively transparent or opaque in the volume-rendered image.
When rendering vascular images, for example, a VRT transfer function is typically set to show the high intensity opacified lumen of a vessel. Viewing a vessel lumen is a useful function that generally helps radiologists detect pathological conditions such as aneurysm or stenosis, but it only tells part of the story. Much of the essential information is not carried in the lumen geometry, but in the contents of the region immediately surrounding the lumen. In healthy vessels, for example, the lumen is immediately adjacent to the vessel wall. In other cases, however, other materials may collect between the vessel lumen and the vessel wall, such as plaque and thrombus, for example. Visualization, measurement and categorization of such other materials would be useful to a clinician.
Typically, plaque and thrombus may be represented in computed tomograpic angiography images (CTA), but other imaging modalities such as magnetic resonance (MR), computed tomography (CT), rotational 3D angiography (3D Angio) and 3D/4D ultrasound may also represent this material. It is currently possible to visualize the lumen alone by adjusting the transfer function in a volume-rendered image. Unfortunately, it is extremely difficult, if not impossible, to simultaneously and globally visualize plaque and lumen automatically, or even semi-automatically, using such a transfer function. Accordingly, what is desired is an automated way to visualize volume-rendered data having occlusions.